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Today lots of us are concerned about autism, and its apparently rising incidence. Writing for Discover magazine, the biologist Emily Willingham takes on a contentious question: Is autism really on the rise, or are we just diagnosing it more? A survey of the field, she writes, shows that "the disorder hasn’t actually become much more common -- we’ve just developed better and more accurate ways of looking for it."

It's certainly true, Willingham writes, that the number of children diagnosed with autism has risen sharply -- a new report from the CDC says that as many as 1 in 88 children may have autism. But an increase in the number of diagnoses might not mean that the number of people with autism is actually increasing. Instead, it may just reflect changes in way developmental issues are diagnosed.

"Leo Kanner first described autism almost 70 years ago, in 1944," Willingham points out. "Before that, autism didn’t exist as far as clinicians were concerned, and its official prevalence was, therefore, zero. There were, obviously, people with autism, but they were simply considered insane." As the diagnosis became widely utilized, of course, the number of cases increased. Doctors were able to see that people who had been given other diagnoses would be better-described by a diagnosis of autism. First, in the 1950s, patients diagnosed as schizophrenic were re-diagnosed as autistic. More recently, children who had been diagnosed as "mentally retarded" or "language impaired" have been re-diagnosed:

Several decades after the introduction of autism as a diagnosis, researchers have reported that professionals are still engaging in “diagnostic substitution”: moving people from one diagnostic category, such as “mental retardation” or “language impairment,” to the autism category. For instance, in one recent study, researchers at UCLA re-examined a population of 489 children who’d been living in Utah in the 1980s. Their first results, reported in 1990, identified 108 kids in the study population who received a classification of “challenged” (what we consider today to be “intellectually disabled”) but who were not diagnosed as autistic. When the investigators went back and applied today’s autism diagnostic criteria to the same 108 children, they found that 64 of them would have received an autism diagnosis today, along with their diagnosis of intellectual disability.

That kind of change is called "diagnostic substitution," Willingham explains, and it accounts for a lot of the rise in autism diagnoses. And there's a lot of other, corroborating evidence, too. If autism were on the rise, you'd expect there to be more autistic children that autistic adults. Adults, though, don't tend to be screened for autism -- and when you do screen them, you find that the prevalence of autism is about the same among adults as it is among kids (1%). That suggests that autism isn't on the rise. And that prevalence is the same all over the world, despite the fact that that the environmental factors which often take the blame for the rise in autism are unevenly distributed.

About brainiac Brainiac is the daily blog of the Globe's Sunday Ideas
section, covering news and delights from the worlds of art, science,
literature, history, design, and more. You can follow us on Twitter @GlobeIdeas.

Guest blogger Simon Waxman is Managing Editor of Boston Review and has written for WBUR, Alternet, McSweeney's, Jacobin, and others.

Guest blogger Elizabeth Manus is a writer living in New York City. She has been a book review editor at the Boston Phoenix, and a columnist for The New York Observer and Metro.

Guest blogger Sarah Laskow is a freelance writer and editor in New York City. She edits Smithsonian's SmartNews blog and has contributed to Salon, Good, The American Prospect, Bloomberg News, and other publications.

Guest blogger Joshua Glenn is a Boston-based writer, publisher, and freelance semiotician. He was the original Brainiac
blogger, and is currently editor of the blog HiLobrow, publisher of a series of
Radium Age science fiction novels, and co-author/co-editor of several books,
including the story collection "Significant Objects" and the kids'
field guide to life "Unbored."

Guest blogger Ruth Graham is a freelance journalist in
New Hampshire, and a frequent Ideas contributor. She is a former features
editor for the New York Sun, and has written for publications including Slate
and the Wall Street Journal.

Joshua Rothman is a graduate student and Teaching Fellow in the Harvard English
department, and an Instructor in Public Policy at the Harvard Kennedy
School of Government. He teaches novels and political writing.